Laminate filter media

ABSTRACT

The present invention provides a self-supporting laminate filter medium having an electret lofty spunbond web and an electret microfiber web, wherein the spunbond web has a density between about 0.01 g/cm 3  and about 0.1 g/cm 3 .

This is a continuation application of U.S. patent application Ser. No.08/577,955 filed Dec. 22, 1995, now U.S. Pat. No. 5,721,180.

BACKGROUND OF THE INVENTION

The present invention is related to a filter medium. More specifically,the present invention is related to a filter medium for gaseous fluids.

Filter media having large interfiber pores and, thus, a highpermeability typically contain sparsely packed relatively thick fibers.Such filter media require relatively low driving pressure to provideadequate filtration throughput rate and extended service-life. However,highly permeable filter media, e.g., residential glass fiber HVACfilters, only provide a low filtration efficiency in that the largeinterfiber pore structures of the media do not have interstitialconfigurations that are suitable for entrapping fine contaminantparticles. Consequently, coarse fiber filter media have not been used infine particle filtration applications.

In contrast, microfiber nonwoven webs, such as meltblown fiber webs,have been used as fine particle filter media. The densely packed finefibers of these webs provide fine interfiber pore structures that arehighly suitable for mechanically trapping or screening fine particles.However, the fine pore structure of meltblown fiber webs and othersimilar microfiber webs that have densely packed fine fibers results ina low permeability, creating a high pressure drop across the webs.Consequently, the low permeability of fine fiber filter media requires ahigh driving pressure to establish an adequate filtration throughputrate. Furthermore, as contaminants accumulate on the surface of thefilter media, the contaminants quickly clog the small interfiber poresand further reduce the permeability of the media, thereby even furtherincreasing the pressure drop across the media and rapidly shortening theservice-life.

Additionally, microfiber web filter media do not tend to have a physicalintegrity that is sufficient enough to be self-supporting. Although thephysical integrity of microfiber filter media can be improved byincreasing the basis weight or thickness thereof, the increased basisweight or thickness exacerbates the pressure drop across the filtermedia. As such, microfiber web filter media are typically laminated to asupporting layer or fitted in a rigid frame. However, the conventionalsupporting layer or rigid frame does not typically contribute to thefiltration process and only increases the production cost of the filtermedia.

There remains a need for self-supporting filter media that providecombinations of desirable filter properties, including high filtrationefficiency, high permeability, low pressure drop, high throughput rateand long service-life.

SUMMARY OF THE INVENTION

The present invention provides a laminate filter medium having anelectret lofty spunbond web and an electret microfiber web, wherein thespunbond web has a density between about 0.01 g/cm³ and about 0.1 g/cm³.The invention also provides a laminate filter medium containing anelectret lofty spunbond web and an electret meltblown fiber web, whereinthe conjugate filaments have at least one polyolefin component polymerand the meltblown fibers have a polyolefin component. The lofty spunbondweb of the filter medium contains crimped multicomponent conjugatespunbond filaments, and the lofty spunbond web has a density betweenabout 0.01 g/cm³ and about 0.1 g/cm³. Additionally provided is a processfor filtering a gas medium with the laminate filter medium of thepresent invention.

The filter media of the invention provide highly advantageous filterproperties including high filter efficiency and high capacity or longservice-life, making the media highly useful for, e.g., various HVAC andcombustion engine filter applications.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE illustrates an electretizing process suitable for the presentlaminate filter media layers.

DETAILED DESCRIPTION OF THE INVENTION

There is provided in accordance with the present invention aself-supporting laminate filter medium having a high filter efficiencyand a long service-life. The filter medium contains at least one layerof an electretized lofty spunbond nonwoven web and at least one layer ofan electretized microfiber web. The laminate filter medium is highlysuitable for gaseous filtration applications, e.g., HVAC filters.

The lofty layer is characterized in that it contains crimped continuousfilaments and that the filaments form substantially evenly distributedinterfiber bonds throughout the web such that the lofty layer has alarge interfiber void volume and a low density. The interfiber bonds ofthe web are formed where the filaments make contact with one another,especially at cross-over contact points. In accordance with theinvention, the lofty spunbond web layer has a density between about 0.01g/cm³ and about 0.1 g/cm³, desirably between about 0.015 g/cm³ and about0.075 g/cm³, and more desirably between about 0.02 g/cm³ and about 0.05g/cm³. In addition, the lofty layer, which has a large interfiber voidvolume, desirably has a Frazier permeability of at least about 100 ft³/minute/ft² (cfm/sf), desirably between about 100 cfm/sf and about 2500cfm/sf, more desirably between about 150 cfm/sf and about 2000 cfm/sf,as measured in accordance with Federal Test Method 5450, Standard No.191A. It is believed that the low-density and porous structure of thelofty layer provide numerous but tortuous paths for gas to traveltherethrough and, thus, provide highly suitable means for mechanicallyand electrostatically trapping particulates or contaminants, therebyproviding a high filter efficiency without a high pressure drop acrossthe filter media. In addition, the evenly distributed interfiber bondsof the lofty layer impart high physical integrity and strength in thelayer, thereby making the lofty layer and the laminate filter mediacontaining the lofty layer self-supporting.

Desirable lofty nonwoven webs suitable for the lofty layer includenonwoven webs containing crimped multicomponent conjugate spunbondfilaments, i.e., crimped multicomponent conjugate spunbond webs. Theterm "multicomponent conjugate filaments" as used herein indicatesfilaments containing at least two different component polymers that arearranged to occupy distinct sections across the cross-section of each ofthe filaments along the entire or substantially entire length thereof.The term "spunbond filaments" as used herein indicates small diameterfilaments that are formed by extruding one or more molten thermoplasticpolymers as filaments from a plurality of capillaries of a spinneret.The extruded filaments are cooled while being drawn by an eductive orother well-known drawing mechanism to form spunbond filaments. The drawnspunbond filaments are then deposited or laid onto a forming surface ina random manner to form a loosely entangled and uniform fiber web. Thelaid fiber web is then subjected to a bonding process to impart physicalintegrity and dimensional stability. Typically, spunbond filaments havean average diameter of at least about 10 μm. Exemplary processes forproducing spunbond nonwoven webs are disclosed, for example, in U.S.Pat. Nos. 4,340,563 to Appel et al., 3,802,817 to Matsuki et al.,3,855,046 to Hansen et al. and 3,692,618 to Dorschner et al.

In accordance with the present invention, the conjugate filaments of thelofty layer contain at least two component polymers having differentmelting points, and the lowest melting component polymer forms at leasta portion of the peripheral surface of each of the filaments. Thecomponent polymers desirably are selected to have a melting pointdifference between the highest melting component polymer and the lowestmelting component polymer of at least about 5° C., more desirably atleast about 10° C., most desirably at least about 30° C., such that thelowest melting polymer can be melted or rendered tacky without meltingthe higher melting component polymers of the filaments. The differencein melting point is advantageously used to bond nonwoven webs containingthe conjugate filaments. When a nonwoven web containing the conjugatefilaments is heated to a temperature equal to or higher than the meltingpoint of the lowest melting component polymer but below the meltingpoint of the highest melting component polymer, the melted peripheralportions of the filaments form interfiber bonds, especially at thecross-over contact points, throughout the web while the high meltingpolymer portions of the filaments maintain the physical and dimensionalintegrity of the web.

The multicomponent conjugate filaments suitable for the lofty layer arecrimped to form a lofty nonwoven web. Suitable filaments for the presentfilter media have at least about 2 crimps per extended inch (2.54 cm),desirably between about 2 and about 50 crimps per extended inch, moredesirably between about 3 and about 30 crimps per extended inch, asmeasured in accordance with ASTM D-3937-82. The filaments can be crimpedbefore or after the filaments are deposited to form a nonwoven web.However, as a specific embodiment of the present invention, it is highlydesirable to crimp the filaments before they are deposited to form anonwoven web in order to ensure dimensional stability and uniformity ofthe web. This is because filaments inevitably change their dimensionswhen crimps are imparted therein, and it is highly impractical tocontrol the movement of crimping filaments to protect againstdimensional and uniformity changes in the web, which inevitablyaccompany the crimping process.

A particularly suitable process for producing crimped multicomponentspunbond webs useful for the present lofty layer is disclosed in U.S.Pat. No. 5,382,400 to Pike et al., which patent in its entirety isherein incorporated by reference. Briefly, a particularly desirableprocess for producing a multicomponent conjugate spunbond web includesthe steps of melt-spinning continuous multicomponent conjugatefilaments, at least partially quenching the multicomponent filaments sothat the filaments have latent crimpability, drawing the filaments andactivating the latent crimpability by applying heated drawing air, andthen depositing the crimped, drawn filaments onto a forming surface toform a nonwoven web. In general, a higher drawing air temperatureresults in a higher number of crimps, provided that the temperature isnot so high as to heat the filaments to a temperature above the meltingpoint of the lowest melting component polymer of the filaments. Inaccordance with this process, the multicomponent conjugate filamentshave a conjugate filament configuration that is amenable for thermalcrimping processes. For example, a conjugate filament having twocomponent polymers (bicomponent filaments) may have a side-by-side oreccentric sheath-core cross-sectional configuration.

The nonwoven web formed from the spunbond conjugate filaments issubsequently bonded using any effective bonding means that heats the webto a temperature sufficiently high enough to melt the lowest meltingcomponent polymer but below the melting point of the higher meltingstructural component polymers of the web, thereby causing the filamentsto form interfiber bonds, especially at cross-over contact points,throughout the web. For example, a through-air bonding, oven bonding, orinfrared bonding process that effects interfiber bonds without applyingsignificant compacting pressure can be used. Particularly suitable ofthese is a through-air bonding process which effects interfiber bonds bythoroughly and evenly heating the web with a penetrating flow of forced,heated air.

The conjugate filaments of the lofty layer are produced from a widevariety of thermoplastic polymers that are known to form fibers.Desirably, the thermoplastic polymers have a resistivity greater thanabout 10¹³ ohms-cm, more desirably greater than about 10¹⁴ ohms-cm, asmeasured in accordance with ASTM 257-61. As indicated above, theconjugate filaments contain at least two component polymers havingdifferent melting points. In accordance with the present invention, atleast one of the component polymers is selected from polymers that areelectretizable and form a highly durable electret. Particularly suitableelectretizable polymers include polyolefins. Examples of suitablepolyolefins include polyethylene, e.g., high density polyethylene, lowdensity polyethylene and linear low density polyethylene; polypropylene,e.g., isotactic polypropylene, syndiotactic polypropylene, and blends ofisotactic polypropylene and atactic polypropylene; polybutene, e.g.,poly(1-butene) and poly(2-butene); polypentene, e.g., poly(1-pentene),poly(2-pentene), poly(3-mehtyl-1-pentene) and poly(4-methyl-1-pentene);copolymers thereof, e.g., ethylene-propylene copolymers; and blendsthereof. Polymers suitable for the other component polymers of theconjugate filaments include above-illustrated polyolefins; polyamides,e.g., nylon 6, nylon 6/6, nylon 10, nylon 12 and the like; polyesters,e.g., polyethylene terephthalate, polybutylene terephthalate and thelike; polycarbonates; polystyrenes; thermoplastic elastomers, e.g.,ethylene-propylene rubbers, styrenic block copolymers, copolyesterelastomers and polyamide elastomers and the like; fluoropolymers, e.g.,polytetrafluoroethylene and polytrifluorochloroethylene; vinyl polymers,e.g., polyvinyl chloride; polyurethanes; and blends and copolymersthereof.

In accordance with the present invention, particularly suitableconjugate filaments are bicomponent filaments, and particularlydesirable pairs of component polymers include polyolefin-polyamide,e.g., polyethylene-nylon 6, polyethylene-nylon 6/6, polypropylene-nylon6, polypropylene-nylon 6/6, polyethylene-a copolymer of nylon 6 andnylon 6/6, and polypropylene-a copolymer of nylon 6 and nylon 6/6;polyolefin-polyester, e.g., polyethylene-polyethylene terephthalate,polypropylene-polyethylene terephthalate, polyethylene-polybutyleneterephthalate and polypropylene-polybutylene terephthalate; andpolyolefin-polyolefin, e.g., polyethylene-polypropylene andpolyethylene-polybutylene. Of these pairs, more particularly desirableare polyolefin-polyolefin pairs, e.g., linear low densitypolyethylene-isotactic polypropylene, high density polyetylene-isotacticpolypropylene and ethylene-propylene copolymer-isotactic polypropylene.

In accordance with the present invention, the laminate filter medium hasat least one microfiber web layer in addition to the lofty layer.Desirably, the basis weight of the microfiber web layer of the laminatefilter media is between about 7 g/m² (gsm) and about 100 gsm, moredesirably between about 10 gsm and about 70 gsm. The microfiber weblayer of the filter medium is characterized in that it containsrelatively closely distributed microfibers. Particularly desirablenonwoven webs for the microfiber web layer of the present invention aremeltblown fiber webs. The term "meltblown fibers" as used hereinindicates fibers formed by extruding a molten thermoplastic polymerthrough a plurality of fine, usually circular, die capillaries as moltenthreads or filaments into a high velocity gas stream which attenuate thefilaments of molten thermoplastic polymer to reduce their diameter. Asis known in the art, the flow rate and pressure of the attenuating gasstream can be adjusted to form continuous meltblown filaments ordiscontinuous fibers. The formed air-borne fibers, which are not fullyquenched, are carried by the high velocity gas stream and deposited on acollecting surface to form a web of randomly dispersed and autogenouslybonded meltblown fibers. An exemplary process for producing meltblownfiber web is disclosed in U.S. Pat. No. 3,849,241 to Butin et al. Ingeneral, microfibers, especially meltblown fibers, have an average fiberdiameter of up to about 10 μm. Desirably, microfibers suitable for themicrofiber layer have an average fiber diameter between about 1.5 μm andabout 8 μm, more desirably between about 2 μm and about 6 μm.

The microfiber layer of the laminate-filter media can be produced from awide variety of thermoplastic polymers that are electretizable and forma highly durable electret. Particularly suitable electretizable polymersinclude polyolefins, such as the polyolefins illustrated above inconjunction with the conjugate filaments.

In accordance with the present invention, both the lofty layer and themicrofiber web layer are electretized. Electret treating processessuitable for the present invention are known in the art. These methodsinclude thermal, plasma-contact, electron beam and corona dischargemethods, and electretizing processes can be applied during the fiberspinning stage of the nonwoven web forming process or after the nonwovenweb is fully formed. For example, U.S. Pat. No. 4,215,682 to Kubik etal. discloses an electretizing process for meltblown fibers that impartsa permanent electrostatic charge during the fiber spinning process, andU.S. Pat. Nos. 4,375,718 to Wadsworth et al. and 5,401,446 to Tsai etal. disclose electretizing processes for fully formed nonwoven webs.

The individual layers of the laminate fiter media or the filter mediacan be conveniently electretized by sequentially subjecting the web to aseries of electric fields such that adjacent electric fields havesubstantially opposite polarities with respect to each other. Forexample, one side of web is initially subjected to a positive chargewhile the other side is subjected to a negative charge, and then thefirst side of the web is subjected to a negative charge and the otherside of the web is subjected to a positive charge, imparting permanentelectrostatic charges in the web. A suitable apparatus for electretizingthe nonwoven web is illustrated in FIG. An electretizing apparatus 10receives a nonwoven web 12 having a first side 14 and a second side 15.The web 12 passes into the apparatus 10 with the second side 15 incontact with guiding roller 16. Then the first side 14 of the web comesin contact with a first charging drum 18 which rotates with the web 12and brings the web 12 into a position between the first charging drum 18having a negative electrical potential and a first charging electrode 20having a positive electrical potential. As the web 12 passes between thecharging electrode 20 and the charging drum 18, electrostatic chargesare developed in the web 12. A relative positive charge is developed inthe first side and a relative negative charge is developed in the secondside. The web 12 is then passed between a negatively charged second drum22 and a positively charged second electrode 24, reversing thepolarities of the electrostatic charge previously imparted in the weband permanently imparting the newly developed electrostatic charge inthe web. The electretized web 25 is then passed on to another guidingroller 26 and removed from the electretizing apparatus 10. It is to benoted that for discussion purposes, the charging drums are illustratedto have negative electrical potentials and the charging electrodes areillustrated to have positive electrical potentials. However, thepolarities of the drums and the electrodes can be reversed and thenegative potential can be replaced with ground. In accordance with thepresent invention, the charging potentials useful for electretizingprocesses may vary with the field geometry of the electretizing process.For example, the electric fields for the above-described electretizingprocess can be effectively operated between about 1 KVDC/cm and about 30KVDC/cm, desirably between about 4 KVDC/cm and about 20 KVDC/cm, whenthe gap between the drum and the electrodes is between about 1.2 cm andabout 5 cm. The above-described suitable electretizing process isfurther disclosed in above-mentioned U.S. Pat. No. 5,401,446, which inits entirety is herein incorporated by reference.

The layers of the laminate filter media of the present invention can beadjoined by various means that intimately juxtapose the layers together.For example, the layers can be bonded to have uniformly distributed bondpoints or regions. Useful bonding means for the present inventioninclude adhesive bonding, e.g., print bonding; thermal bonding, e.g.,point bonding; and ultrasonic bonding processes, provided that theselected bonding process does not alter, e.g., diminish, thepermeability or porosity of the web layers or the interface of thelayers to a degree that makes the laminate undesirable for its intendeduse. Alternatively, the layers can be bonded only at the peripheraledges of the media, relying on the pressure drop across the media duringuse to form juxtaposed laminates. As yet another alternative, the layerscan be sequentially formed on a forming surface. For example, a loftyspunbond layer is formed on a forming surface, and then the lofty layeris conveyed under a meltblown web-forming apparatus and a meltblownfiber web layer is directly formed on the lofty layer, thereby forming afirmly attached laminate filter medium.

The basis weight of the laminate filter media may vary widely. However,particularly suitable filter media have a basis weight from about 10 gsmto about 500 gsm, more particularly from about 14 gsm to about 450 gsm,and most particularly from about 15 gsm to about 340 gsm. In accordancewith the invention, the filter media contain between about 5 wt % and 95wt % of the lofty layer and between about 95 wt % and 5 wt % of themicrofiber web layer, based on the total weight of the filter media.Desriably, the filter media contain between about 50 wt % and 94 wt % ofthe lofty layer and between about 60 wt % and 6 wt % of the microfiberweb layer.

The laminate filter media of the present invention provide a high filterefficiency and a long service-life. Surprisingly, it has been found thatthe lofty layer and the microfiber web layer of the laminate filtermedia synergistically improve the filter efficiency while substantiallymaintaining the long service-life of the lofty layer. The filter mediaare highly suitable for HVAC filters, combustion engine filters and thelike that require high filtration throughput rate and relatively lowpressure drop across the filter media.

The following examples are provided herein as illustration of theinvention, and the scope of the present invention is not limitedthereto.

EXAMPLES Test Procedures Used

NaCl Filter Efficiency Test

This test method determines filter retention of sodium chlorideparticles in an apparatus that sends a flow of NaCl aerosol particlessuspended in air at a rate of 5 ft/min. into a 0.5 ft² filter medium.The NaCl particles were generated into an aerosol from a 1% NaClsolution by a Laskin nozzle type atomizer, and the particle size rangewas between approximately 0.1 μm and 3 μm. The efficiency of the filtermedium was determined by measuring the particle size distribution andnumber of particles at positions upstream and downstream of the testfilter medium. The efficiency was defined as 100*(1-(downstream particlecounts/upstream particle counts)). The particle sizes and counts weremeasured using an automatic particle counter and sensor, HIAC/ROYCOModel 5109/1230, which are available from Pacific Scientific Co., SilverSpring, Md.

Filter Pressure Drop

A fresh filter medium was placed in the above NaCl filter efficiencytesting apparatus, and the pressure drop across the filter medium wasmeasured in mm of water. The measured pressure drop is the pressuredifference between the influent stream and the effluent stream acrossthe filter medium.

ASHRAE 52.1 Filter Efficiency Test

This test measures the efficiency of a filter medium with a standardizedASHRAE dust. The test procedure was similar to the NaCl test, excepttest dust particles were injected into the air stream and a HIAC/ROYCOModel 8000 automatic particle counter was used. The ASHRAE dustcontained 72% standard AC fine, 23% powder carbon black and 5% cottonlinters. The ASHRAE test was conducted on a 1 square feet (0.093 m²)filter medium at a higher air flow rate than the NaCl test, and the airflow rate used was 25 ft/min.

Frazier Permeability

The Frazier permeability, which expresses the permeability of a fabricin terms of cubic feet per minute of air per square foot of medium at apressure drop of 0.5 inch (1.27 cm) of water, was determined utilizing aFrazier Air Permeability Tester available from the Frazier PrecisionInstrument Company and measured in accordance with Federal Test Method5450, Standard No. 191A.

Density

The density of each filter medium was calculated from the basis weightand the caliper, which was measured at 3.5 g/cm² (0.05 psi) with aStarret-type bulk tester.

Example 1 (Ex1)

A low density through-air bonded spunbond web containing bicomponentconjugate filaments was produced in accordance with the procedureoutlined in aforementioned U.S. Pat. No. 5,382,400. The bicomponentspunbond web contained linear low density polyethylene-polypropyleneconjugate spunbond filaments and had a basis weight of 102 gsm (3.0 osy)and a density of 0.039 g/cm³. Linear low density polyethylene, Aspun6811A, which is available from Dow Chemical, was blended with 2 wt % ofa TiO₂ concentrate containing 50 wt % of TiO₂ and 50 wt % of apolypropylene, and the mixture was fed into a first single screwextruder. Polypropylene, PD3443, which is available from Exxon, wasblended with 2 wt % of the above-described TiO₂ concentrate, and themixture was fed into a second single screw extruder. The extrudedpolymers were spun into round bicomponent fibers having a side-by-sideconfiguration and a 1:1 weight ratio of the two polymers using abicomponent spinning die, which had a 0.6 mm spinhole diameter and a 6:1L/D ratio. The temperatures of the molten polymers fed into the spinningdie were kept at 450° F., and the spinhole throughput rate was 0.5gram/hole/minute. The bicomponent fibers exiting the spinning die werequenched by a flow of air having a flow rate of 0.5 m³ /min/m² (45SCFM/inch) spinneret width and a temperature of 18° C. (65° F.). Theaspirator was equipped with a temperature controlled aspirating airsource, and the feed air temperature was kept at about 177° C. (350°F.). The fibers for each test specimen entering the aspirator were drawnwith the heated feed air at a flow rate of 19 ft³ /minute/inch width.The weight-per-unit-length measurement of the drawn fibers was about 3denier per filament (3.3 dtex). The drawn fibers were then deposited ona foraminous forming surface with the assist of a vacuum flow to form anunbonded fiber web. The unbonded fiber web was bonded by passing the webon a foraminous supporting surface through a through-air bonder thatapplied a flow of heated air at a temperature of 133° C. (272° F.) and avelocity of 30.5 m/min (200 feet/min). The residence time for each webspecimen in the bonder was about 2-4 seconds. The bonded nonwoven webswere charged by passing the web at a speed of 100 ft/min through anelectretizing apparatus that contained two sections. The first sectionof the electretizing apparatus had a wire electrode, which was placedabove the web and had a positive potential of about 16 KV, and a roller,which was placed below the web and was grounded; and the second sectionhad a charging roller, which was placed above the web and had a negativepotential of about 7.5 KV, and a wire electrode, which was placed belowthe web and had a positive potential of about 25 KV. The gap between thecharging electrode and the roller was kept at about 2.54 cm (1 inch).

A 10 gsm (0.3 osy) polypropylene meltblown web was produced inaccordance with the process described in U.S. Pat. No. 3,978,185 toButin et al. The polypropylene was Himont's HH441. The meltblown web waselectretized in accordance with the above-described process.

A layer of the electretized lofty spunbond web and a layer of theelectretized meltblown web were cut to 28 cm by 36 cm rectangles. Thetwo layers were placed in the NaCl filter efficiency testing apparatus,placing the lofty layer toward the influent side. The laminate filtermedium was tested for various filter properties. The results are shownin Table 1.

Comparative Example 1 (C1)

The lofty electretized spunbond web of Example 1 was tested for itsfilter properties. The results are shown in Table 1.

Comparative Example 2 (C2)

The electretized meltblown web of Example 1 was tested for its filterproperties. The results are shown in Table 1.

Comparative Example 3 (C3)

A 20 gsm (0.6 osy) meltblown web was prepared in accordance with theprocess described in U.S. Pat. No. 3,978,185 to Butin et al., and thepolymer used was Himont's PS015 polypropylene. The web was electretizedby following the electretizing process outlined in Example 1. Theelectretized meltblown web was tested for its filter properties. Theresults are shown in Table 1.

                  TABLE 1    ______________________________________                Ex1  C1        C2     C3    ______________________________________    Basis Wt. (osy)                  3.3    3.0       0.3  0.6    (g/m.sup.2)   112    102       10   20    Density (g/cm3)                  --     0.030     --   --    Filter Pressure                  0.7    0.23      0.4  1.3    Drop (mm H.sub.2 O)    Frazier Per-  56     131       92   27    meability    (m.sup.3 /min/m.sup.2)    Filter Efficiency                  90     70        63   85    NaCl Test (%)    ______________________________________

As can be seen from the filter efficiency results of Example 1 andComparative Examples 1-3, the combination of the lofty layer, C1, andthe microfiber layer, C2, synergistically improves the filter efficiencywithout unduely increasing the filter pressure drop. In addition, thefilter efficiency, pressure drop and Frazier permeability data ofExample 1 and Comparative Example 3 clearly demonstrate that thelaminate filter media of the present invention not only havesignificantly improved filter efficiency over microfiber web filtermedia but also do not require the high driving pressure of themicrofiber web filter media.

Example 2 (Ex2)

A three-layer laminate filter medium was prepared. The filter medium hada layer of a 17 gsm (0.5 osy) point bonded spunbond web, a middle layerof a 54 gsm (1.6 osy) meltblown web and a layer of a 102 gsm (3.0 osy)lofty spunbond web. The point bonded spunbond web was prepared inaccordance with U.S. Pat. No. 3,855,046 to Hansen et al. using PD3443polypropylene, and the web was pattern bonded with a diamond pattern of225 bonds per square inch (35 bonds/cm²) covering about 25% of thesurface area. The meltblown web and the lofty conjugate filamentspunbond web were prepared in accordance with the processes described inExample 1. The lofty spunbond layer and the meltblown layer wereelectretized in accordance with the procedure outlined in Example 1.Then the three layers were cut to 28 cm by 36 cm rectangles. Then thethree layers were positioned in a laminate form, and the completeperipheral edge of the laminate was thermally bonded.

The filter medium was tested for various filter properties including itsfilter capacity and efficiency. The filter capacity, which correspondsto the filter service-life, was tested using the ASHRAE filterefficiency test setup. The efficiency test was run until the pressuredrop across the medium reached 2.54 cm (1.0 inch) H₂ O. The filtermedium was removed from the apparatus and the weight gain was measured.The weight gain indicates the filter capacity of the medium.

Comparative Example 4 (C4)

A three-layer laminate filter medium was prepared as in Example 2,except the lofty spunbond web was replaced with a 92 gsm (2.7 osy) airlaid nonwoven web. The air laid nonwoven web was obtained bydelaminating the air laid nonwoven web layer of an industrial HVACfilter which is available from Hollings and Bose. The air laid nonwovenweb was not an electret web.

The filter property tests were conducted in accordance with Example 2.The results are shown in Table 2.

Comparative Example 5 (C5)

The electretized meltblown layer of Example 2 was tested for its filterproperties. The results are shown in Table 2.

Comparative Example 6 (C6)

The electretized lofty spunbond layer of Example 2 was tested for itsfilter properties. The results are shown in Table 2.

Comparative Example 7 (C7)

The air laid nonwoven layer of Comparative Example 4 was tested for itsfilter properties. The results are shown in Table 2.

                  TABLE 2    ______________________________________             Ex2   C4      C5      C6     C7    ______________________________________    Basis Wt. (osy)               5.1     4.8     1.6   3.0    2.7    (g/m.sup.2)               173     163     54    102    92    Density (g/cm3)               --      --      --    0.029  0.036    Frazier Per-               --      --      --    152    152    meability    (m.sup.3 /min/m.sup.2)    Filter Efficiency               94.4    90.6    --    --     --    ASHRAE (%)    Filter     14.2    5.2     --    --     --    Capacity (g)    ______________________________________

Despite the fact that the laminate filter media of Example 2 andComparative Example 4 had similar physical properties, e.g., similarbasis weight, permeability, porosity and construction, the filter mediumof the present invention exhibited highly superior filter efficiency andfilter capacity over the Comparative Example 4 filter medium. The onlydifference between the two filter media of Example 2 and ComparativeExample 4 was that the Example 2 filter medium contained an electretizedlofty spunbond layer of the present invention whereas the ComparativeExample 4 contained a nonelectret air laid web layer. This resultclearly demonstrate that the electret lofty spunbond web layer of thepresent invention in combination with the electret microfiber web layersynergistically improves the filter efficiency of the laminate filtermedium.

What is claimed is:
 1. A method of making a laminate filter mediacomprising:forming a multicomponent spunbond fiber web having at least afirst thermoplastic polymer component and a second thermoplastic polymercomponent wherein said first thermoplastic polymer component has ahigher melting point than said second thermoplastic polymer component;through-air bonding said multicomponent spunbond fiber web wherein saidmulticomponent spunbond fiber web has inter-fiber bonds at fibercross-over points distributed substantially throughout saidmulticomponent spunbond fiber web and wherein the density of said bondedmulticomponent spunbond fiber web is between about 0.01 and 0.1 g/cm³ ;electret treating said bonded multicomponent spunbond fiber web whereinsaid multicompnent spunbond fibers retain a charge; forming a microfiberweb; electret treating said microfiber web wherein said microfiber webretains a charge; and fixedly attaching said bonded multicomponent webto said microfiber web to form a contiguous multilayer laminate whereinthe central portion of said multilayer laminate has a substantiallyuniform thickness and air permeability.
 2. The method of claim 1 whereinsaid bonded multicomponent web and said microfiber web are bonded alonga peripheral edge of said multilayer laminate.
 3. The method of claim 2wherein said bonded multicomponent fiber web and said microfiber web arethermally bonded along said peripheral edge of said multilayer laminate.4. The method of claim 1 wherein said bonded multicomponent fiber weband said microfiber web are adhesively bonded along a peripheral edge ofsaid multilayer laminate.
 5. The method of claim 1 wherein said bondedmulticomponent fiber web and said microfiber web are ultrasonicallybonded along a peripheral edge of said multilayer laminate.
 6. Themethod of claim 2 wherein said webs are laminated to form a multilayerlaminate having a Frazier Air Permeability of at least 100 ft.³/min./ft.².
 7. The method of claim 6 wherein said multilayer laminatehas a ASHRAE filter efficiency of at least 90%.
 8. The method of claim 7wherein said spunbond filter media has a density between 0.015 and 0.075g/cm³.
 9. The method of claim 1 wherein said microfiber web is formeddirectly over said multicomponent spunbond fiber web thereby forming asubstantially cohesive multilayer laminate.
 10. The method of claim 9wherein said microfiber web comprises a meltblown fiber web.
 11. Themethod of claim 10 wherein said multilayer laminate is further bondedalong a peripheral edge.
 12. The method of claim 10 wherein said websare laminated to form a multilayer laminate having a Frazier AirPermeability of at least 100 ft.³ /min./ft.².
 13. The method of claim 12wherein said multilayer laminate has a ASHRAE filter efficiency of atleast 90%.
 14. The method of claim 13 wherein said spunbond filter mediahas a density between 0.015 and 0.075 g/cm³.